1. What happened
'MapMagic World Generator' demo scene fails to generate terrain in 2017.1.0b1. There's DemoScene that, on Play, stucks with 'Generating Terrain' status, throwing errors to console:

Assertion failed: Assertion failed on expression: 'outputVertexCount == sharedData.vertexCount'

Assertion failed: Assertion failed on expression: 'outputIndexCount == sharedData.indexCount'

IndexOutOfRangeException: Array index is out of range.
(wrapper managed-to-managed) object:ElementAddr_3_4 (object,int,int,int)
MapMagic.WeldTerrains.WeldSplats (System.Single[,,] splats, UnityEngine.Terrain prevX, UnityEngine.Terrain nextZ, UnityEngine.Terrain nextX, UnityEngine.Terrain prevZ, Int32 margin) (at Assets/MapMagic/Main/WeldTerrains.cs:136)
...

This same scene works fine in 5.6.0p1. But opened either new or updated from 5.6, fails in 2017.1.0b1

2. How we can reproduce it using the example you attached
- Import 'MapMagic World Generator' A$ package into new project
- Open DemoScene
- Run
---> Observe errors in Console